In high precision spectroscopy and laser cooling of atomic Lithium vapors, a laser source or “pumping laser” is typically required ideally having the following specifications: power within a range of hundreds of mW; relatively continuous wave radiation (i.e., not pulsed), with generally low amplitude noise; a frequency tunable over a number of Ghz at a wavelength corresponding generally to the atomic resonance of Lithium (671 nm); a relatively small spectral linewidth relative to 6 MHz (6 MHz being the natural linewidth of atomic transition); a single transverse mode, such as Gaussian, with an M2 lower than at least about 1.5, that ensures at least around a 50% coupling efficiency for a single transverse mode optical fiber; a relatively low cost; simple operation, an efficiency generally stable and reliable for at least a few months of regular use; and a power dissipation of not higher than a few kW.
Although there are many optical conversion systems in the art, none fully satisfies the “ideal” requirements, as set forth above, for high precision spectroscopy and laser cooling of atomic Lithium vapors. In the case of conventional semiconductor-type laser sources, the associated laser diodes have been found suitable for operating in a neighborhood of 671 nm, and supplying power up to between about 5 W and about 10 W. These relatively high power diodes have an emission spectrum that is many THz wide, and their spatial beam profile is multimode. To satisfy the desired criteria of the spectral linewidth and beam profile mode, however, low power laser diodes (20 mW maximum power) stabilized on extended cavity in the Littrow, or Littman-Metcalf configuration, must be used. With these configurations, the maximum available power is about 50% of the initial power from the diode laser. Hence, the total power obtainable is no more than about 10 mW. Generally speaking, it is possible to amplify low power radiation with good spectral properties through injection locking of slave laser diodes, or by use of seeding semiconductor tapered amplifiers (See, e.g., “A high-power multiple-frequency narrow-linewidth laser source based on a semiconductor tapered amplifier”, G. Ferrari et al., Optics Letters 24, 151 (1999)). Notwithstanding, by using a laser diode as slave a amplifier, it is possible to achieve no more than about 20 mW, semiconductor tapered amplifiers for a wavelength lower than approximately 730 nm being ready available on the market.
Another conventional laser source is the dye laser. Dye lasers, at least theoretically, represent a viable alternative, particularly in terms of their flexibility of use. By varying the type of dye used and wavelength of the pumping laser, it is possible to generate radiation having an emission wavelength ranging from infrared to ultraviolet. In the case of 671 nm radiation, it is possible to generate such radiation using systems on the market, for instance, the Coherent 699 Dye Laser, with Rhodamine dye. By this combination, the pumping laser must have an emission wavelength generally within a range of 500 nm and 550 nm, for which the most common choice is an Argon ion (Ar+) laser, or a Neodymium or Ytterbium YAG laser having a frequency doubling stage (515 nm or 532 nm). While useful, the efficiency of the Argon ion (Ar+) laser is considered rather low (about 0.1%), hence to generate power within a range of a few Watts, it is necessary to dissipate many kW of energy. Also, this laser has been found very unreliable, typically has a non-negligible amplitude and pointing noise, and quite expensive both to purchase and maintain, requiring frequent realignment of the cavity. For these reasons, Argon ion lasers have not generally found widespread industrial application.
Alternatives to the Ar+ pumping laser include a frequency doubled Nd:YAG or Yb:YAG laser. These lasers have been found relatively reliable (normally requiring replacement of the pumping diode bars only every 10,000 working hours), and are quite efficient (e.g. to produce 10 W of radiation, the overall power consumption, even after taking into account the Nd/Yb:YAG bar cooling system, is typically only around 1 kW). While dye lasers also meet the precision spectroscopy and laser cooling criteria for output power, spectral purity, and spatial mode, they are not only expensive, but can also be complicated to operate.
Another option is a Titanium-Sapphire (Ti:Sa) laser. These lasers not only have spectral characteristics and working conditions similar to those of dye lasers for 671 nm, but also have much greater simplicity of operation. While useful, the Ti:Sa gain curve is centered at around 850 nm, and their application at 671 nm (on the tail of the gain profile) is not considered efficient. Moreover, the cost of a Ti:Sa laser, like a dye laser, is relatively high.
Since there are no known laser sources emitting directly at the required wavelength, both at an acceptable cost and in a fully satisfactory manner of operation, laser sources may also be considered that rely on non-linear frequency conversion, such as Second Harmonic Generation (SHG), Frequency Sum, and Optical Parametric Oscillator (OPO).
Second Harmonic Generation is a conversion process that effectively doubles the frequency of radiation, by sending the fundamental radiation through a crystal that is non-linearly polarizable and simultaneously satisfies the phase-matching conditions (or quasi-phase-matching, if periodically poled crystals are used) between the fundamental radiation and that generated via the Second Harmonic Generation process. Currently, there are non-linear crystals available that are considered well-suited for Second Harmonic Generation toward the complete visible spectrum, near ultraviolet, and near infrared, are available. More specifically, crystals are available that generate about 671 nm light from 1342 nm. Hence, the pumping laser must still deliver at least around 1 W at about 1342 nm, as well as suitable spectral purity, frequency tunability and spatial mode quality. Although semiconductor sources deliver up to few tens of mW of power, amplifiers (e.g., Raman fiber amplifiers) are considered unsuitable for generating radiation having a linewidth lower than about 1 Ghz. Another possible alternative is the Neodymium Vanadate laser. Unfortunately, however, these lasers are not available in a continuous wave (CW) version, and do not satisfy the criteria of frequency tunability.
Frequency Sum is a process where two distinct base photons, each having a different frequency, are combined or added together so as to generate a single photon, the frequency of which is the sum of the frequencies of the two base photons. For applications at 671 nm, a laser at one micron may be used (like a laser diode stabilized on an extended cavity) that is amplified up to about 5 W on a Yb fiber amplifier, and about a 2 micron laser (i.e., a laser diode amplified on a Thulium fiber amplifier, up to around 10 W available). Although this provides power that may be rather high for efficient non-linear conversion, an optical cavity must be used that is doubly resonant with radiation at about 1 and 2 microns. While this is not problematic for radiation at approximately 1 micron, problems may arise with radiation at around 2 microns, principally due to a lack of detectors (i.e., for optical alignment, and making the radiation resonant with one of the cavity modes) efficient at this wavelength. In addition, this solution requires that both lasers be summed or combined in order to fulfill the requirements of spectral purity, power, and spatial mode quality, thereby doubling of the complexity of the laser source.
As for the Optical Parametric Oscillator (or OPO), it is a device that, beginning at a laser field of frequency f1, produces two fields having frequencies of f2 and f3, respectively, such that f2+f3=f1. The OPO essentially comprises an optical cavity housing a non-linear crystal that satisfies the conditions of phase-matching, or quasi-phase-matching, for the process f1f2+f3. The starting laser having a frequency f1 is referred to as the “pump”. If the frequency f2 is higher than f3, then f2 is called the “signal” and f3 the “idler”. An OPO is known as singly resonant when the optical cavity is resonant with one field (“signal” or “idler”), doubly resonant when the resonance is both with the “signal” and “idler”, and triply resonant when the pump is resonant with the “signal” and “idler” as well. The simplest method for generating 671 nm radiation with an OPO begins with a “pump” at a frequency higher than about c/671 nm=446 THz, thereby directly producing around 671 nm. Beginning with a “pump” at 532 nm, then combining the same with a “pump” at 671 nm, radiation at approximately 2568 nm is produced.
Realistically, an OPO of this sort would be singly resonant with an operating threshold of pumping laser power at around 2-3 W (an arrangement of this general description is disclosed, for instance, in “Continuous-wave singly-resonant optical parametric oscillator based on periodically poled LiNbO3”, Bosenberg et al., Optics Letters 21, 713 (1996)). Because spectral characteristics of the radiation so generated depend substantially on experimental conditions, it is conceivable to achieve at least some of the precision spectroscopy and laser cooling requirements described previously, namely, producing around 100 mW of radiation at about 671 nm using a 10 W pumping laser at around 532 nm. To achieve this, it is apparent that a relatively powerful, 532 nm pumping laser must be employed, albeit with associated relatively high cost. Considering the power available from these systems, the intrinsic frequency instability (which depends generally on the stability of the cavity, the crystal, and the pumping laser), and the overall cost, even this solution is not considered satisfactory, at least realistically, for industrial application.
A last option in the art is a variation of the above-described OPO, namely, an optical parametric oscillator in combination with other non-linear processes such as Frequency Sum or Second Harmonic Generation. This approach seeks to combine the flexibility of the wavelength generated by the OPO with processes of frequency duplication of the generated field (i.e., Second Harmonic Generation), or Frequency Sum, with the pump field (see, e.g., “Frequency up conversion by phase-matched sum-frequency generation in an optical parametric generator”, E. C. Cheung et al., Optics Letters 19, 1967 (1994)). Despite the advantage of generating radiation of relatively higher frequency, these systems have similar features to and, thus, the disadvantages of, traditional OPOs. Accordingly, this approach has also been found unsatisfactory for applications at around 671 nm.